+Interaction with mutexes
+
+ One must be careful to do not cause deadlocks due to interaction of
+ synchronize_rcu() and RCU read-side with mutexes. If synchronize_rcu()
+ is called with a mutex held, this mutex (or any mutex which has this
+ mutex in its dependency chain) should not be acquired from within a RCU
+ read-side critical section.
+